1. Field of the Invention
The present invention relates to an abnormality detection apparatus for a power feed circuit that is built in a vehicle-mounted electronic control apparatus (an engine control apparatus, a transmission control apparatus, etc.), and more particularly, it relates to such an abnormality detection apparatus for a power feed circuit in which a power transistor for controlling the supply of power to an electric load has an overcurrent limiting function and an overheat interruption function, and which can identify and determine the presence or absence of an open-circuit abnormality and a short-circuit abnormality of the electric load in a quick manner.
2. Description of the Related Art
In general, in power feed control on an electric load, there have been widely put into practical use intelligent power switching devices having a power transistor, an overcurrent limiting function and an overheat interruption function.
Specifically, there has been proposed an apparatus which can limit an overcurrent through switching (on/off) control of a main transistor by the use of a current detection resistor arranged in a current mirror circuit, and detect an overheat of the main transistor by using a temperature detection element, so that the current supplied to the electric load is interrupted by an N-MOS type field-effect transistor (see, for example, a first patent document: Japanese patent application laid-open No. 2000-312142 (FIG. 1 and Abstract)).
Also, as an abnormality detection apparatus for a power feed circuit (a failure detection apparatus for a solenoid) which performs power feed control on a solenoid (electric load) by means of a transistor, there has been proposed an apparatus which can identify and determine the presence or absence of an open-circuit or break abnormality and a short-circuit abnormality by making a comparison between a switching control command for a transistor and a power feed voltage level for the electric load (see, for example, a second patent document: Japanese patent application laid-open No. H8-293414 (FIG. 2 and Paragraph Nos. 0024 through 0028)).
According to the conventional abnormality detection apparatus for a power feed circuit, for example, in the case of the intelligent power switching device as described in the above-mentioned first patent document, the energization of an overheat self-hold type semiconductor switch (power transistor) connected to a downstream side of the electric load is interrupted upon occurrence of an overcurrent, but no particular reference is made to how to handle an open-circuit abnormality or a short-circuit abnormality of the power feed circuit, and an open-circuit state and a short-circuit state of the electric load cannot be detected in a quick manner, thus posing a problem that the power transistor is not able to be protected to a satisfactory extent.
In addition, in the case of the failure detection apparatus for a solenoid described in the above-mentioned second patent document, a determination as to whether the solenoid is in an open-circuit abnormality or in a short-circuit abnormality is made, irrespective of the operating state of the solenoid, by artificially energizing the solenoid in a short time during an inoperative period thereof or artificially interrupting the energization of the solenoid in a short time during an operating period thereof, but no particular reference is made to how to handle an overcurrent limiting characteristic and a transition characteristic of an abnormal state detection signal, and hence it is impossible to detect an open-circuit state and a short-circuit state of the electric load in a quick manner, as in the case of the first patent document. Thus, in this case, too, there is a problem that the power transistor is not able to be protected to a satisfactory extent.